The present invention generally relates to plant cutting devices, in particular vegetation cutters or line trimmers, and more particularly, to a novel cutting head configuration and filament for such a device.
For many years, various forms of cutting strings or filaments have been developed that are designed to be mounted in a cutting head, driven by an electric motor or an engine in a manner such that the rapidly rotating head drives the filament(s), which tend(s) to take up a rectilinear configuration under the centrifugal force. Such filaments are generally made of polyamide by an extrusion process.
Whereas the older filaments were of circular cross-section and were smooth over their entire lengths, filaments have now been developed that can be of various cross-sections, intended to improve the quality of cutting (presence of edges, etc.) and/or the longevity of the filament, and/or intended to reduce the noise of the device when in operation, by acting on turbulence.
Thus, filaments exist that are provided with edges, grooves, channels, or local deformations of the filament along its length.
Unfortunately, a drawback that is common to all known filaments is that the angular positioning of the filament at its working length is generally ill-controlled, so that, when a filament is supposed to engage the plants at a cutting edge, for example, there is no guarantee that such an edge will indeed be the leading edge that hits the plants first.
An example representative of that problem lies in filaments of approximately square cross-section. In which case, the plants might be engaged, at least locally, by one of the plane faces interconnecting two consecutive edges, instead of by an edge of the filament.
And even though attempts have been made to develop filaments whose cutting effectiveness is improved compared with the cutting effectiveness obtained with a circular cross-section, and is independent of the real angular positioning of the filament, it can be observed that the gain in effectiveness remains extremely limited.
Document WO-A-99/40773 discloses a cutting unit with a cutting filament having an airfoil shaped profile which operates in a predetermined angular position. However, such a cutting filament requires a special manufacturing process, in which the filament must be twisted through about 90° at well-determined places in order to ensure that the airfoil profile does indeed work in the desired angular position.
Unfortunately, such an approach is incompatible with current mass production techniques using extrusion, and in which very long lengths of filament are manufactured that are then cut up into reels or strands of desired length, but in which any manufacturing action on the strands one after the other seriously jeopardizes the manufacturing cost.
Another airfoil profile is disclosed by Document U.S. Pat. No. 5,761,816, in which angular positioning of the filament is guaranteed by shaping the opening in the cutting head for passing the filament so that said opening has the same profile as the filament.
U.S. Pat. No. 5,048,278 also discloses a cutting filament of triangular cross-section that is held in a predetermined angular position where it is attached to the cutting head. Unfortunately, the cutting unit in that document suffers from two major drawbacks. Firstly, mounting the filament in the cutting head to maintain the angular positioning of the filament requires extremely complex mechanical means and extremely complex handling. Secondly, the angular position of the filament of triangular section as it is described in that document is not capable of guaranteeing optimum cutting.
Finally, U.S. Pat. No. 4,118,865 describes another cutting head having a triangular filament which, however, requires a special provision to be made, namely a washer crimped at the root of the filament in order to attach it to the cutting head, thereby complicating manufacture and making it more costly. In addition, the angular positioning of the cross-section of the filament relative to that fastening washer is not accurately defined, since fastening is apparently by manual crimping.
It is an object of the present invention to mitigate the above drawbacks of the state of the art, and to provide a cutting device in which, while using a cutting filament that is made uniformly over a very long length, in particular by extrusion, it is nevertheless possible to ensure that a cutting filament provided with a cutting edge works in a specific angular position, in which the cutting edge offers its optimum effectiveness.